Ideas to tinker

[Tinkerer]

While it is AC and arguably bipolar it is not bipolar-symmetric. There is a slow ramp dB/dt that is uniformly positive, and a fast ramp dB/dt that is uniformly negative. You will normally sample either ramp response in an additive manner, meaning that Earth field effect is not naturally canceled. I suppose there is a way to combine a response sample from the fast negative and the slow positive to cancel EFE but that imposes a severe restriction on how you can do target processing.

Normally the answer would be to add a late EFE sample and subtract it. In this case the TX is always active (exponential) and so the target response is also always active. However, if you consider the slow ramp to be perfectly linear then a fast target will probably be substantially decayed near the end of the ramp and EFE subtraction will work OK. But a high conductor response will be substantially canceled.

Disclaimer: Having never built this circuit or even done much investigation on it, I will admit that this is speculation. Feel free to shoot me down.

Thank you for your help, Carl.
Your analysis is very helpful and you have pointed out many features. I can agree with most.

This waveform is however very deceptive in it?s simplicity.

I believe the waveform can be made to be much more symmetric and to look more linear, but it will always stay exponential. There are in fact 4 exponentials in this waveform.
The target will add at least one more exponential superposed on each of the 4 exponentials.

As the TX current is constantly changing, it induces eddy currents in the target at all times.

The good thing about that, is that it is possible to sample the waveform at all times, during the positive half cycle and during the negative half cycle.
At different times, different information about the target can be gleaned. The bad thing is that there are so many choices to be made of where to look. One really needs to decide on a specific purpose to know what to look for.

And then there are the dead spots. The Zero crossings. Again, they are not symmetric. More complications.
We will go down this rabbit hole another time.

[Carl-NC]

While it is AC and arguably bipolar it is not bipolar-symmetric. There is a slow ramp dB/dt that is uniformly positive, and a fast ramp dB/dt that is uniformly negative. You will normally sample either ramp response in an additive manner, meaning that Earth field effect is not naturally canceled. I suppose there is a way to combine a response sample from the fast negative and the slow positive to cancel EFE but that imposes a severe restriction on how you can do target processing.

Normally the answer would be to add a late EFE sample and subtract it. In this case the TX is always active (exponential) and so the target response is also always active. However, if you consider the slow ramp to be perfectly linear then a fast target will probably be substantially decayed near the end of the ramp and EFE subtraction will work OK. But a high conductor response will be substantially canceled.

Disclaimer: Having never built this circuit or even done much investigation on it, I will admit that this is speculation. Feel free to shoot me down.

[Tinkerer]

Thanks for the reply. My simulation uses a coil that I have. Using my coil specs, reduced PS to 6V and added a 10nf capacitor across coil similar to what you did and it seems to work.

The capacitor rating needs to be AC, and considerably higher Voltage than the Flyback. Film works good, but Ceramic also works, but needs a footprint for high Voltage.

Any amount of inductance works. Then adjust the timing and/or the capacitor.

Bundle wound magnet wire works fine.

You can use high repetition rate or low.

[green]

Attached is the simulation with a few changes to make it fit the Mosfet, without avalanching.

With the desired design parameters, we could design it specifically.

Thanks for the reply. My simulation uses a coil that I have. Using my coil specs, reduced PS to 6V and added a 10nf capacitor across coil similar to what you did and it seems to work.

[Tinkerer]

Now I look at it from a different angle:

During half of the cycle the current flows into the coil.
The other half of the cycle the current flows out of the coil.

[Tinkerer]

Zero to 500mA, -500mA to zero. Both halves, increasing slope.

I can agree with that.

[green]

HA HA, Maybe I should have said "arguably bi-polar"? If I look at one half cycle, I see a triangular wave from zero Amps to +500mA.
The second halve period goes from zero Amps to minus 500mA. Just like an AC wave goes. Is AC bi-polar?

Any opinions?

Zero to 500mA, -500mA to zero. Both halves, increasing slope.

[Tinkerer]

Even if made symmetric it's still not bipolar. I'm just looking a way to eliminate EF cancellation.

HA HA, Maybe I should have said "arguably bi-polar"? If I look at one half cycle, I see a triangular wave from zero Amps to +500mA.
The second halve period goes from zero Amps to minus 500mA. Just like an AC wave goes. Is AC bi-polar?

Any opinions?

[eclipse]

Even if made symmetric it's still not bipolar. I'm just looking a way to eliminate EF cancellation.

[Tinkerer]

Do you think it's possible to get a bipolar TEM with a single coil? Since current is flowing through the whole TX cycle maybe that's not possible? What do you think?

How much bi-polar do you want it to be?

I added a Zero Amp line to the picture. Positive on top, negative below.

Not quite symmetric? Play with the parameters to make it symmetric.

However, You still need an RX coil and a way to reduce the TX pulse on the RX coil. There are several methods to do that. DD, concentric with bucking coil, passive compensating coil, Active controlled compensation coil, etc. You can also find several patents, some old and some more recent of how to do that.

[eclipse]

Do you think it's possible to get a bipolar TEM with a single coil? Since current is flowing through the whole TX cycle maybe that's not possible? What do you think?

[Tinkerer]

TEM TX CURRENT

This is what the TX current looks like. A deceptively simple saw-tooth wave.

In fact it is very complex. We will look at the complex details in days to come.

[Tinkerer]

Attached is the simulation with a few changes to make it fit the Mosfet, without avalanching.

With the desired design parameters, we could design it specifically.

[green]

https://www.geotech1.com/forums/atta...9&d=1596044550
Right side circuit(TEM)isn't correct. Spice simulation doesn't avalanche MOSFET so coil decay is faster than should be.

[Tinkerer]

philosophy

Before we get deeper into the circuits, I would like to talk a bit about my philosophy of tinkering.

We started off with a traditional PI TX circuit, "in the box". The limiting factors are the coil turns and inductance, coil capacitance and peak flyback voltage. Basically the "brick walls" of the box.

for people less familiar with the "jargon": a "brick wall" is a big problem. An insurmountable problem. No way over it, no way around it no way under it.
So here comes the philosophy again:

Every problem is also an opportunity. An opportunity to solve the problem.

Going back to the PI TX capacitance problem: Everything has been tried to reduce the capacitance. Over the recent years some progress has been made, but we are at the "brick wall" of the PI TX capacitance.

The opportunity? More about that later.